P-xylene (hereinafter referred to as PX) and propylene are both important and valuable raw materials essential for chemical industry. At present, p-xylene is mainly obtained by an aromatic hydrocarbon combination apparatus. A reformate containing aromatic hydrocarbons is prepared by continuous reforming of naphtha, and a PX product is then maximally obtained via units of aromatic extraction, aromatic fractional distillation, disproportionation and transalkylation, xylene isomerization, and adsorptive separation, etc. Since the content of p-xylene among three isomers is thermodynamically controlled, and p-xylene comprises only about 23% in C8 mixed aromatic hydrocarbons, the amount of recycling process is large, equipment is bulky, and the operational cost is high during the whole PX production process. Particularly, the differences between boiling points of three isomers of xylene are very small, high-purity p-xylene cannot be obtained with typical distillation techniques, and an expensive process for adsorptive separation has to be used. Propylene is mainly derived from byproducts in petroleum refineries and also from the production of ethylene by steam cracking of naphtha, or is produced by using propane as a raw material, which is prepared by processing of natural gas. p-Xylene is mainly used in the production of polyesters, and propylene is extremely useful in the preparation of polypropylene and acrylonitrile as well as 1,3-propylene glycol required in the production of polyesters. The rapid development of the global economy increasingly demands for p-xylene and propylene as essential chemical feedstock.
In recent years, there are a number of domestic and foreign patents that disclose new routes for the production of p-xylene, and among these, methylation of toluene may produce p-xylene with a high selectivity. U.S. Pat. No. 3,965,207 discloses that a ZSM-5 molecular sieve is used as a catalyst to perform methylation reaction of toluene, wherein the highest selectivity of p-xylene at a reaction temperature of 600° C. is about 90%; U.S. Pat. No. 3,965,208 uses a ZSM-5 molecular sieve modified with VA element as a catalyst, and the generation of m-xylene is inhibited and p-xylene and o-xylene are mainly generated, wherein the highest selectivity of p-xylene at a reaction temperature of 600° C. is about 90%; U.S. Pat. No. 4,250,345 uses a ZSM-5 molecular sieve modified with two elements phosphorus and magnesium as a catalyst, wherein the optimal selectivity of p-xylene at a reaction temperature of 450° C. is up to 98%; U.S. Pat. No. 4,670,616 prepares a catalyst by using a borosilicate molecular sieve and silicon oxide or aluminum oxide, wherein the selectivity of p-xylene is 50-60%; U.S. Pat. Nos. 4,276,438, and 4,278,827 use a molecular sieve having a special structure (SiO2/Al2O3≥12), which is modified with copper, silver, gold, germanium, tin, lead, etc., and a p-dialkylbenzene with a high selectivity can be obtained. U.S. Pat. No. 4,444,989 uses a crystalline pure silicon molecular sieve, which is modified with compounds of arsenic, phosphorus, magnesium, boron and, tellurium, and the selectivity of p-xylene is improved. U.S. Pat. No. 4,491,678 uses combined components of a crystalline borosilicate and elements of Group IIA and IIIA as well as silicon and phosphorus, and the selectivity of p-xylene may be greatly improved and the service life of the catalyst can be improved. U.S. Pat. No. 5,034,362 uses ZSM-5 and ZSM-11 wherein SiO2/Al2O3≥12 as catalysts and performs calcination under a condition of higher than 650° C., and the selectivity of p-dialkylbenzenes may be improved. U.S. Pat. No. 5,563,310 uses an acidic molecular sieve containing an element of Group IVB, which is modified with a metal of Group VIB, and the selectivity of p-dialkylbenzenes in alkylation reaction of toluene and methanol may be improved. U.S. Pat. No. 6,504,072 uses a mesoporous molecular sieve, preferably ZSM-5, which is treated in steam higher than 950° C. and then modified with phosphorus oxides, and proposes the influence of the diffusion effect of catalyst micropores on the selectivity of p-xylene. U.S. Pat. No. 6,613,708 uses an organic metal compound to modify a catalyst, and the selectivity of p-dialkylbenzenes may be greatly improved.
Chinese Patents ZL200610011662.4, ZL200710176269.5, ZL 200710176274.6, ZL 200710179408.X, ZL 200710179409.4, and ZL 200710179410.7 disclose a class of methods for preparing p-xylene and co-producing light olefins from toluene and methanol, indicating that ethylene and propylene may be co-produced with a high selectivity at the same time of preparing p-xylene with also a high selectivity, wherein the selectivity of p-xylene in xylene isomers is up to 99 wt % or more and the selectivities of ethylene and propylene in C1-C5 light hydrocarbons may be 90 wt % or more. However, the disadvantages of this method are that a cryogenic separation technique has to be used if a highly pure ethylene product is to be obtained, and that the investment and energy consumption are both high, which directly affects the economy of this process.
CN102464549 A discloses a method for producing propylene and p-xylene, comprising preparing propylene by the comproportionation of ethylene and C4 hydrocarbons, wherein the process for preparing propylene by the alkylation of ethylene and methanol/dimethyl ether is not involved. CN102464550 A discloses a method for co-production of light olefins and p-xylene, comprising preparing olefins by passing C4 and C5 hydrocarbons into a first reaction zone, which is a process of preparing olefins by cracking of C4 or liquefied gas, wherein the process for preparing propylene by the alkylation of ethylene and methanol/dimethyl ether is not involved either.